Why has humanity never been visited by aliens (that we know of)? The question has confounded scientists for decades, but two researchers have come up with a possible — and disturbing — explanation: Advanced civilizations could be doomed to either stagnate or die before they get the chance.

The new hypothesis suggests that, as space-faring civilizations grow in scale and technological development, they eventually reach a crisis point where innovation no longer keeps up with the demand for energy. What comes next is collapse. The only alternative path is to reject a model of “unyielding growth” in favor of maintaining equilibrium, but at the cost of a civilization’s ability to expand across the stars, the researchers said.

The argument, published on May 4 in the journal Royal Society Open Science(opens in new tab), attempts to find a resolution to the Fermi Paradox. Taking its name from the casual lunchtime musings of Nobel Prize-winning physicist Enrico Fermi, the paradox draws attention to the contradiction between the immense scope and age of the universe — two things that suggest the universe should be teeming with advanced alien life — and the lack of evidence that extraterrestrials exist anywhere in sight. “So where is everybody?” Fermi is thought to have remarked.

“Civilizations either collapse from burnout or redirect themselves to prioritizing homeostasis(opens in new tab), a state where cosmic expansion is no longer a goal, making them difficult to detect remotely,” astrobiologists Michael Wong, of the Carnegie Institution for Science, and Stuart Bartlett, of the California Institute of Technology, wrote in the study. “Either outcome — homeostatic awakening or civilization collapse — would be consistent with the observed absence of [galactic-wide] civilizations.”

The pair came to their hypothesis by researching studies of the “‘superlinear”‘ growth of cities. These studies suggested that cities increase in size and energy consumption at an exponential rate as their populations grow, inevitably leading to crisis points — or singularities — that cause rapid crashes in growth, followed by an even more precipitous, potentially civilization-ending, collapse.

“We hypothesize that once a planetary civilization transitions into a state that can be described as one virtually connected global city, it will face an ‘asymptotic burnout,’ an ultimate crisis where the singularity-interval time scale becomes smaller than the time scale of innovation,” they wrote.

These close-to-collapse civilizations would be the easiest for humanity to detect, the researchers suggest, as they would be dissipating large amounts of energy in a “wildly unsustainable” way. “This presents the possibility that a good many of humanity’s initial detections of extraterrestrial life may be of the intelligent, though not yet wise, kind,” the researchers wrote.

To avert their doom, civilizations could undergo a “homeostatic awakening,” redirecting their production away from unbounded growth across the stars to one that prioritizes societal wellbeing, sustainable and equitable development and harmony with their environment, the researchers suggest. While such civilizations may not completely abandon space exploration, they would not expand on scales great enough to make contact with Earth likely.

The researchers point to a few of humanity’s “mini-awakenings” that addressed global crises on Earth, such as the reduction of global nuclear arms stockpiles from 70,000 warheads to below 14,000; the halting of the once-growing hole in Earth’s ozone(opens in new tab) layer by banning chlorofluorocarbon emissions; and the 1982 international whaling moratorium.

The scientists stress, however, that their suggestion is simply a hypothesis, taken from the observation of laws that seem to govern life on Earth, and is designed to “provoke discussion, introspection and future work.”

Their proposal joins a bountiful crop of other scientific and popular suggestions as to why we’ve never made direct contact with celestial visitors. These include the numerous practical challenges presented by interstellar travel; that aliens may actually be visiting in secret; or that aliens arrived to Earth too soon (or humans too early) in the life of the universe for direct contact.

Another hypothesis, published April 4 in The Astrophysics Journal(opens in new tab), suggests that the sheer scale of the universe means it could take as long as 400,000 years for a signal sent by one advanced species to be received by another — a timescale that’s far greater than the brief period humans have been able to scan the skies.